RFR (S) 8148936: Adapt UUID.toString() to Compact Strings

[Ivan Gerasimov]

Hi Aleksey!

Looks good to me!

Two minorish comments

1)
431             formatUnsignedLong0(lsb >> 48, 4, buf, 19, 4);
I would suggest using unsigned shift >>> here just to avoid thinking 
about whether the sign matters or not.
Not a big deal, of course.

2)
Do you want to add the bug id 8148936 to the @bug list in the regression 
test?

Sincerely yours,
Ivan


On 03.02.2016 21:25, Aleksey Shipilev wrote:
> Hi,
>
> JDK-8006627 did the JavaLangAccess hack to improve UUID.toString()
> performance:
>
>      public String toString() {
>          char[] chars = new char[36];
>          jla.formatUnsignedLong(mostSigBits >> 32, 4, chars, 0, 8);
>          chars[8] = '-';
>          jla.formatUnsignedLong(mostSigBits >> 16, 4, chars, 9, 4);
>          chars[13] = '-';
>          jla.formatUnsignedLong(mostSigBits, 4, chars, 14, 4);
>          chars[18] = '-';
>          jla.formatUnsignedLong(leastSigBits >> 48, 4, chars, 19, 4);
>          chars[23] = '-';
>          jla.formatUnsignedLong(leastSigBits, 4, chars, 24, 12);
>          return jla.newStringUnsafe(chars);
>      }
>
> This is a good performance improvement, but it clashes with Compact
> Strings which now have to re-compress the resulting char[] array into
> byte[]. And we know that UUID would always produce Latin1 String.
>
> In fact, the entire JavaLangAccess.newStringUnsafe is now not doing what
> power users would expect: it *does* allocate now! So, we need to phase
> out that internal gateway to avoid confusion. UUID is one of these users
> (StringJoiner is another).
>
> This is the proposed fix:
>    http://cr.openjdk.java.net/~shade/8148936/webrev.01/
>
> My attempts in exposing the entire String coder business to UUID proved
> to be rather ugly, so I opted to just all into a single method, and let
> java/lang internals to sort this out.
>
> The patch does restore the post-Compact Strings performance, and even
> improves it further. See:
>    http://cr.openjdk.java.net/~shade/8148936/notes.txt
>
> Cheers,
> -Aleksey
>
>